what's the local (i.e. destination) port? Source ports are most of the arbitrary ports.Nowadays networks are quite noisy, all kinds of discovery attempts, UPnP, Bonjour, printers, media protocols, you name it. The traffic is very likely "normal" but OTOH not essential for normal operation.

Guess my next question would be harder to answer - "Why is Bobs PC trying to snoop on Daves PC?" - This local port 1900 seems to be plug n' play broadcasting to all PCs. I can't imagine why this would need to happen so perhaps the approach of 'block it and see what breaks'? (Obviously only on a very small number of PCs to begin with)

as to why, ("modern") devices are looking for available services on the network, printers, media servers, and so on. This is normally not directed traffic (e.g.Bob's PC snooping Dave's PC), the source sends packets to a broadcast address and devices on the LAN reply (or don't). You can look up the port at IANA - if the Description doesn't already tell what it is (17500 - Dropbox LanSync Discovery) it shouldn't be too hard to find detailed information (e.g. look up the Description on Wikipedia).